In ground leveling work or grading work performed over a wide range of area, a bulldozer is generally used. In this case, the more the range of area increases, the more the leveling control in ground finishing work or in grading work is important. Consequently, heretofore, it is usual to perform the ground finishing work with reference to a reference plane which is measured each time the ground finishing work is performed after the bulldozer performs the primary ground leveling work (hereinafter referred to as the "first conventional method").
On the other hand, in recent years, a second conventional method has been also developed for performing the ground leveling work or the grading work based on a reference plane which is formed by scanning a work area or ground with a laser beam light issued from a rotary laser projector installed in the work area.
In the second conventional method, the rotary laser projector is rotatably driven to form a horizontal optical reference plane or an oblique optical reference plane inclined at an arbitrary angle. A photo receiver for receiving a laser beam light issued from the laser projector is mounted on a bulldozer, and serves as a ground-level detecting unit for detecting a level of the ground relative to the optical reference plane to issue a level signal to a control unit of the bulldozer, so that a position of a blade of the bulldozer is automatically controlled based on the level signal to perform ground leveling work or grading work in an appropriate manner.
However, the above first conventional method is tedious and time consuming, and is poor in finishing quality of the ground leveling work.
On the other hand, the second conventional method suffers from a problem in that, since the ground-level detecting unit is directly mounted on the blade of the bulldozer so as to control a position of a cutting edge of the blade serving as a level target in the ground leveling work during which a tractor (which is a main vehicle body portion of the bulldozer) pitches considerably, a level signal or value issued from such ground-level detecting unit extremely varies from that of the optical reference plane during the ground leveling work. In addition, in the second conventional method, the bulldozer is restricted in working speed when its work area includes large concave and convex ground portions. Further, in the second conventional method, when the laser beam light issued from the rotary laser projector is interrupted by the other construction machines such as dump trucks, there is a fear that the position of the blade is not appropriately controlled since the position of the blade is controlled based on the level signal having been received before such interruption.
In addition, heretofore, in the ground leveling work or the grading work performed over a wide area, since it is general for a construction manager to empirically divide the area and empirically decide the execution order of the work in the area, the work is not necessarily performed in an effective manner.